High intensity discharge (HID) lamps typically require the application of a starting voltage or ignition voltage that is substantially higher than the operating voltage of the lamp. This starting voltage must provide a sufficiently higher electric field, such that, in the presence of an avalanche-initiating electron, breakdown will occur. It is well know to those skilled in the art that igniting HID lamps can be difficult, especially in lamps using high buffer gas pressures, in mercury-free lamps or in re-start situations after a lamp has recently been extinguished.
Many attempts have been made to improve the starting of HID lamps. For example, some ignition aids improve the starting performance by assuring the presence of an avalanche-initiating electron. Specifically, the use of UV enhancers and Krypton-85-containing buffer gases are well known. Other methods and devices are intended to enhance the local electric field in the region between the electrodes (or in the discharge volume for electrodeless lamps). Another method of aiding the initiation of a discharge involves increasing the electric field at a give externally applied voltage. It is to the latter category that the instant invention pertains.
Typically, such field enhancement is accomplished by the addition of an electrically conductive member such as a wire or metallized stripe, which reduces the effective arc gap between the electrodes, thus leading to a lower breakdown voltage. The conductor can be floating, as in the case of high pressure sodium lamps, (see, for example, U.S. Pat. No. 6,661,171), or the conductor can be electrically coupled to one of the electrodes. Connection to one of the electrodes introduces an undesirable influence on sodium migration in the case of metal halide or sodium lamps, so a bimetal switch typically is employed to disconnect the starting aid from the electrode as the lamp heats up.
In electrodeless lamps, it has been suggested to embed a conductor into the quartz envelope to provide field enhancement (see, for example, U.S. Pat. No. Re 32,626). The deposition of a matrix coating of conductive and/or semi-conductive fibers has also been suggested to facilitate starting. The deposition can be internal or external and, if internal, it is suggested that the fibers be coated with a sol gel-deposited silica coating to protect the fibers from the plasma environment (see U.S. Pat. No. 6,628,079).
While the above methods have had success in the various large lamps currently in use, the problems of starting high-pressure discharge lamps for automotive headlamps, which require instant on status, are somewhat different.
The conventional approach to assuring instant ignition of high pressure automotive headlamps is to over-voltage the ignition pulse, allowing breakdown to occur with the first or at least the first few ignition pulses to be applied to the lamp by the ballast. This often requires a rapid stream of ignition pulses with peak pulse heights of 20 to 25 kV. The overall goal has been to match the “turn on” speed of conventional halogen incandescent lamps.
Conventional high pressure lamp ballasts, such as those shown in U.S. Pat. No. 6,661,184, apply high voltage starting pulses directly through the two main arc tube electrodes. In addition, the main drive circuitry delivers the sustaining current waveforms directly through the secondary windings of the ignitor step-up transformer. This approach has definite disadvantages from the standpoint of size and heat dissipation. Heavy gauge wire has to be used in the secondary windings to handle the current capacity of the drive circuit. This has the added disadvantage of making the secondary winding rather large, which is a definite drawback for automotive headlamp applications, where it is desired that the lighting systems be as small as possible.
It is known that increasing the frequency of the drive circuitry can significantly reduce ballast size. Higher drive frequency means reduced sizes of some components, mainly those of the inductive components. Unfortunately, the inductance of the secondary windings of the ignitor circuit inhibits the passage of high frequency and prohibits the use of this type of ballast.
An alternative ballast design that eliminates the shortcomings described above is taught by U.S. Pat. No. 5,990,633 to Hirschmann. Therein, the functions of ignition and drive are separated. This is achieved by the use of a third lamp ignition electrode. High voltage pulses are applied from the secondary windings of the ignitor transformer. The ignitor secondary winding is totally removed from the drive circuit in this case and as a result allows for a smaller ignitor transformer, less heat dissipation and higher frequency of operation.
The '633 patent described above teaches that the auxiliary ignition electrode be a thin metallic coating in the form of an elongated strip which extends from the base of the bulb to approximately the center point of the discharge vessel, with the result that the end of the auxiliary electrode remote from the base is approximately the same distance from both electrodes. Also suggested is the use of a thin wire which extends parallel to the longitudinal axis of the lamp or which is looped around the discharge.
It would be an advance in the art if an electrode starting probe could be developed that utilized lower voltage pulse heights from the ignitor.